The physical breaking down of rock is called mechanical weathering, which can occur in several ways. These are dependent on the climate (temperature and precipitation) in an area. The main types of mechanical weathering are as follows: Frost action: This is also known as ice wedging and is caused by the freezing and thawing of water in a crack in the rock. As water freezes, it expands, making the crack larger. After the ice melts, more water can fill the crack. When this water freezes, the crack is again made larger. This cycle keeps happening until the rock breaks apart. Plant action: The growth of plant stems, trunks, and roots through cracks in rock can enlarge the cracks. This can often be seen as weeds grow up through the cracks of sidewalks and roads. Exfoliation: The top layer of rock is peeled off. Glacial advances and retreats can cause the top layer of rock to be removed. Pressure unloading: As the top layer of rock is removed, the bottom layers expand and crack. This is due to a decrease in pressure from the removal of the top layers. As the weight lessens, the rocks expand.